The continuous annealing furnace of a steel sheet, the alloying furnace of a plating facility for steel sheet, and other steel sheet production facilities use induction heating coils for rapidly heating the steel sheets. Such an induction heating coil is made a tubular coil conductor provided with a steel sheet passage inside it (solenoid type), a coil conductor set so as to sandwich the sheet from above and below (traverse type), etc. to enable uniform heating of the steel sheet from the front and back surfaces and is covered on its surface with an insulating material having heat resistance.
As the insulating material for this, in the past, a heat insulating castable refractory or alumina cloth or other alumina ceramic such as high temperature heat-resistant fibers etc. has been used.
Japanese Patent Publication (A) No. 2005-156124 (JP2005-156124 A), unlike the present invention, relates to an induction heating coil of a hot forged material and discloses an insulated structure made of monolithic refractory containing a porous flame resistant aggregate and covering the inside surface of the induction heating coil.
Japanese Patent Publication (A) No. 2006-169603 (JP2006-169603 A) discloses an induction heating coil for the same application of the present invention insulated by an alumina ceramic. However, it does not describe details of the alumina ceramic.
Such a conventional insulated structure of an induction heating coil was selected focusing solely on the heat resistance and insulation ability. It was learned that it was not possible to prevent a drop in insulation due to entry of fine metal particles in the atmosphere (for example, zinc fumes).
In the galvanization line of steel sheet, fine zinc fumes float in the atmosphere in the furnace, so if continuously operating this over a long period, the electromagnetic force causes the zinc fumes to be drawn to and deposit on the insulated surface of the induction heating coil. It was learned that part of this deposits and builds up between the particles, in the cracks, etc. of the insulating material, passes through the insulating material along the cracks, and short circuits the coil surface and the shield plate grounded with the coil or between the coils.
Note that surfaces of the coil wires may also be coated in advance with a varnish, enamel, or other insulating coating. Further, if the temperature inside the furnace exceeds 450° C., these insulating coatings are burned off and the surfaces of the copper wires end up being exposed. For this reason, if zinc fumes enter, it is not possible to prevent a drop in insulation of the induction heating coil.
If such a drop in insulation occurs at the induction heating coil, line stoppages will be invited and serious losses will occur. For this reason, solution of this problem is being strongly sought. Further, to avoid such trouble, it has been necessary to conduct periodic inspections to determine the state of deterioration of the insulation and make repairs.